Calculating stock solution concentration after reconstitution

Lyophilised peptides ship as a dry cake because freeze-drying is the most stable storage form for most sequences. Before it can go into an in-vitro assay, that cake has to become a solution of known concentration — and the only inputs that matter are the mass stated on the vial and the volume of diluent added.

The calculation is simple molarity-style arithmetic: concentration (mg/mL) = total peptide mass (mg) ÷ diluent volume (mL). A 5 mg vial reconstituted with 2 mL of diluent gives a 2.5 mg/mL stock. From there, any working concentration for a plate or culture is a straightforward dilution of that stock — no different from preparing any other reagent at a target concentration.

Two things change the real figure from the one printed on the label. First, the vial's stated mass is nominal; the certificate of analysis gives the gravimetric fill check for that lot, which is the more precise number if you need it. Second, peptide content as a fraction of total powder mass can vary with salt content (acetate, TFA) and residual moisture — the COA's purity figure describes chemical purity of the peptide itself, not powder-to-peptide ratio, so for work where the exact molar concentration matters, accounting for counter-ion mass is worth doing explicitly.

Diluent choice follows solubility, not preference: bacteriostatic water or saline for most sequences, with some requiring a small percentage of acetic acid or DMSO for solubility before dilution into aqueous buffer. Reconstituted stock should be stored at 2–8°C and used within the timeframe appropriate to the sequence's stability — lyophilised powder itself keeps far longer at −20°C than any reconstituted solution.

None of this is usage or administration guidance — it's the same stock-solution arithmetic used for any lab reagent prepared from a dry powder. For strength and fill figures specific to a compound, check its product page and the certificate for the lot in hand.